This invention relates to I.C. engines of the reciprocating-piston type, and is concerned with reducing the level of noise generated by such engines in use. The invention is applicable primarily to automobile engines, although it may also be applied over the whole range of stationary and marine piston engines and even aircraft piston engines.
Overall, considerable reductions in the noise emission levels of piston engines have been achieved by reducing that part of the noise due to the entry of the air used by the engine together with that part due to the release of the exhaust gases to atmosphere via a silenced exhaust pipe system. However, due to the logarithmic nature of the laws governing noise, even when the noise from these two sources has been subtracted from the total there still remains a significant noise level generated by the sum of all the exterior surface movements of the engine, which acts as a three-dimensional loudspeaker diaphragm and puts energy into the surrounding air which the human ear recognises as noise. The resultant wave form of the noise from this source is a complex one having many components of different frequencies, amplitudes and phases derived from the natural frequencies of all the various surfaces and panels, etc. on the exterior of the engine excited by the complex deflections within the load-carrying structure of the engine as a result of the compounded gas and inertia loads imposed on the engine piston and reacted by the cylinder head and cylinder walls.
Many proposals have been made for reducing these noise-generating exterior micromovements of the engine, such as the use of ribbed panels to raise natural frequencies and reduce local amplitudes, and the use of attached exterior panels including elastically-bonded outer panels in which the elastomer itself has a high damping characteristic. In general terms it is necessary to divorce or decouple the outside panels of an engine to the greatest possible extent if movements and resonances, disliked by the ear as noise, are to be avoided.
The present invention approaches the problem of noise reduction by seeking to divorce the large-area crankcase skirt and sump of a piston engine as totally as is practicable from the reaction loads at the engine mounting points, so as to reduce the deflections of these large-area external surfaces under such loads and hence to reduce the generation of radiated noise by them.
It is usual to mount automotive and other piston-type engines on flexible mountings designed to produce low natural frequencies of vibration for the supported engine mass in all modes, thus providing a high degree of isolation of the vehicle chassis or body from unpleasant disturbing vibrations emanating from the engine. In automobiles it is usual to mount the engine and gearbox as a unitary assembly using a single, or sometimes a twin, flexible mounting at the rear of the gearbox and two front flexible mountings attached to the engine crankcase skirt, one on either side of the engine, usually slightly below the crankshaft longitudinal axis, and often approximately midway along the engine length. With existing automotive engines suitable machined pads are provided on and integral with the external crankcase walls, and appropriate flexible mountings, usually of the rubber block type, are bolted to these pads, the lower mounting flanges of the mountings being bolted to the vehicle chassis or body. A very wide range of variation in the design and positioning of these engine mountings is known, but in all cases known to the applicant the engine sides of such mountings are solidly bolted to pads or brackets which are an integral part of the external surface of the engine crankcase or other structure. As a consequence, reacting dynamic forces and couples are transmitted through the external shell of the engine structure, which will as a result deform and possibly resonate, and will emit noise to the surroundings.